1. Field of the Invention
The invention relates to an electronic transmitter for direct current telegraphy systems in which binary data signals are converted into polar line currents for transmission over a telegraph line, the transmitter comprising an input circuit generating command signals from the data signals which are galvanically isolated therefrom, a telegraph voltage source having a positive and a negative pole, a unipolar constant current source, a switching circuit controlled by the command signals for selectively connecting one of the two poles of the telegraph voltage source to the telegraph line through the unipolar constant current source, and an output filter having a capacitor in parallel with the telegraph line.
In the systems for polar direct current telegraphy (also called double-current telegraphy) the formerly generally used transmitters with mechanical telegraph relays are replaced to an increasing extent by transmitters having electronic telegraph relays which offer considerable advantages as regards life, space occupied and price.
If such electronic telegraph transmitters are constructed in the manner mentioned above, they have the additional advantage that they can be used in both current-controlled and in voltage-controlled telegraphy systems. When using these transmitters in current-controlled systems in which the telegraph signals at the input of the telegraph line are defined as currents having a prescribed value (for example 20 mA) and in which the input impedance of the receiver is made as low as possible, the constant current source is adjusted to the prescribed current value. When these transmitters are used in voltage-controlled systems in which the telegraph signals at the input of the telegraph line are defined as voltages having a prescribed value (for example not more than 5 volts less than the telegraphy battery voltage of 80 volts) and in which the input impedance of the receiver is made as large as practically possible, the constant current source is, on the contrary, adjusted to such a high current value (for example 50 mA) that the impedance of telegraph line and receiver together always determine the current at the receiver side and the current source consequently acts as current limiter only.
An important problem which must also get attention when using electronic telegraph transmitters, is crosstalk from the telegraph signals to adjacent transmission lines. Particularly with adjacent telephone lines the interference caused by crosstalk is annoying because it is within the speech frequency band and, consequently, audible. Therefore most telecommunication administrations impose stringent requirements on the level of this type of interference. Since crosstalk increases with frequency the higher frequencies in the telegraph signals should be suppressed to keep the crosstalk interference within acceptable limits.
2. Description of the Prior Art
The networks which were generally used in the past and which are still used with known electronic telegraph transmitters for suppressing the higher frequencies in the telegraph signals, are LC-filters having two series coils and a parallel capacitor or a series coil and two parallel capacitors. In view of the customary telegraph speeds the cut-off frequencies of these LC-filters are rather low, so that the required coils are bulky and therefore considerably limit the possibilities for miniaturisation of the electronic telegraph transmitters.
United Kingdom Pat. Specification No. 1,209,988 discloses the replacement of LC-filters for suppressing higher frequencies in polar signals with substantially instantaneous transitions by a circuit without coils, which circuit comprises a first operational amplifier having such a feedback circuit that the first amplifier supplies an output signal having a substantially linear slope characteristic in response to a transition in the polar signals, and a second operational amplifier having such a feedback circuit that the second amplifier supplies an output signal having a substantially sinusoidal slope characteristic in response to an output signal of the first amplifier. The operation of this circuit is mainly based on the differences in shape of the non-linear current-to-voltage characteristics of silicon diodes and germanium diodes, respectively in the transition range between their conducting and non-conducting states.
Apart from the fact that such a pronounced dependency on the properties of non-linear elements is usually unwanted for practical reasons, the use of this prior art circuit in electronic telegraph transmitters would require drastic changes in the existing transmitter structure which in addition are accompanied by a considerable increase in the number of elements and the power consumption, as this circuit is not suitable for processing the high current and voltage values of the telegraph signals at the transmitter output and, consequently, cannot be applied in the same place as the LC-filters.
Furthermore, it has already been proposed to miniaturize electronic telegraph transmitters by eliminating the coils in the existing LC-filters. However, it then appears that the suppression of the higher frequencies in the telegraph signals is insufficient for keeping the crosstalk interference in adjacent telephone lines within acceptable limits.